jaws: separate initial attribute callbacks from getter locking

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Summary

• define a framework-wide unlocked callback boundary for InitialHTMLAttrHandler; the widget audit found JsVar was the only standard renderer violating it
• split Element.ApplyGetter into tag/event application and a separate Element.ApplyInitialHTMLAttr phase, then migrate HTML, image, input, container, and JsVar renderers
• keep JsVar tag resolution and JSON marshaling in one uninterrupted write-locked snapshot while running initial attributes after unlock
• preserve bound-handler ordering and panic-safe lock release; document that initial attributes need not share the later getter/provider state snapshot
• exercise real callback lock re-entry with both sync.RWMutex and the sync.Mutex bind.AsRWLocker adapter path
• retain bind.Binder synchronization: widgets enter it unlocked, and Binder acquires its own read lock before invoking InitialHTMLAttrHook

Breaking API change

Custom renderers change from:

tagValue, attrs := elem.ApplyGetter(getter)

to:

tagValue := elem.ApplyGetter(getter)
attrs := elem.ApplyInitialHTMLAttr(getter)

The initial-attribute phase must run without an externally held lock on the getter or its source. Tag registration now completes before that explicit phase.

Verification

• go generate ./...
• go vet ./...
• gofmt -l .
• staticcheck ./...
• golangci-lint run
• gosec ./...
• JAWS_REQUIRE_NODE=1 go test -race -coverprofile=coverage.out ./...
• JAWS_REQUIRE_NODE=1 go test ./...
• go build ./...

The Linux-only 386 leg runs in CI; macOS does not support darwin/386.

Fixes #264